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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 6 — Mar. 25, 2013
  • pp: 7641–7650
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Route diversity analyses for free-space optical wireless links within turbulent scenarios

Stanislav Zvanovec, Joaquin Perez, Zabih Ghassemlooy, Sujan Rajbhandari, and Jiri Libich  »View Author Affiliations


Optics Express, Vol. 21, Issue 6, pp. 7641-7650 (2013)
http://dx.doi.org/10.1364/OE.21.007641


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Abstract

Free-Space Optical (FSO) communications link performance is highly affected when propagating through the time-spatially variable turbulent environment. In order to improve signal reception, several mitigation techniques have been proposed and analytically investigated. This paper presents experimental results for the route diversity technique evaluations for a specific case when several diversity links intersects a common turbulent area and concurrently each passing regions with different turbulence flows.

© 2013 OSA

1. Introduction

2. Measurement setup

The experimental measurement set-up using the laboratory atmospheric chamber is depicted in Fig. 1(a)
Fig. 1 (a) Block diagram of the laboratory turbulence chamber; (b) snapshot of the deployment of thermal sensor line inside the chamber.
. At the transmitter side two narrow divergence beam laser sources plus a collimated lens are used. The optical beams are modulated by a data source at a line-rate of 1 Mbit/s. The laboratory atmospheric channel is a closed glass chamber with a dimension of 5.5 × 0.3 × 0.3 m. The chamber has air vents with external fans for air circulation along its length to control the temperature distribution. External heaters are used to pump hot air into the chamber to create the turbulence. The room temperature is around 20 - 25 °C, this range is considered as the cold or the baseline temperature reaching up to 60 °C. There are also 19 remotely controlled thermal sensors positioned along the chamber monitoring and taking measurement of the temperature at every one second interval within a range of −55 °C to + 125°C and a resolution of 0.1 °C, see Fig. 1(b). The receiver front-end consists of a PIN photodetector and a transimpedance amplifier (TIA). The TIA output signal is captured using a wide bandwidth real time digital oscilloscope and a full signal analysis is carried out off line in Matlab. The main parameters of the experimental system are given in Table 1

Table 1. Parameters of the optical wireless Link

table-icon
View This Table
.

The random fluctuation in the atmospheric temperature along the optical beam propagation path results in variation of the atmospheric refractive index nas [28

28. W. K. Pratt, Laser Communication Systems (John Wiley & Sons, 1969).

]. The rate of change of the atmospheric refractive index nas depends on the atmosphere temperature and pressure as given by [29

29. S. Karp, R. M. Gagliardi, S. E. Moran, and L. B. Stotts, Optical Channels: Fibers, Clouds, Water and the Atmosphere (Plenum, 1988).

]:
nas=1+77.6(1+7.52×103λ2)PasTe×106,
(1)
dnas/dTe=7.8×105Pas/Te2,
(2)
where Pas is the atmospheric pressure in millibars, Te is the effective temperature in Kelvin and λ is the wavelength in micrometers. Near the sea level, dnas/dTe106K−1 [29

29. S. Karp, R. M. Gagliardi, S. E. Moran, and L. B. Stotts, Optical Channels: Fibers, Clouds, Water and the Atmosphere (Plenum, 1988).

]. The contribution of humidity to the refractive index fluctuation is not accounted for Eq. (1) because this is negligible at optical wavelengths [30

30. G. R. Osche, Optical Detection Theory for Laser Applications (Wiley-Interscience, 2002).

].

3. Experimental evaluation

3.1 Separated channels

To validate statistical results, the channel separation was increased and measurements were carried out for two separate channels isolated by a divider and foils. The measurement set-up within the turbulence chamber is shown in Fig. 2(b). Channel 2 was influenced by a constant distortion or the impairment due to the intensity variation of the received signal, i.e. with Rytov variance being kept below 0.09. This small intensity variation is not considered to be due to the turbulence and is more to do with the material used to isolate both channels. In this case the foil is a transparent film sheet made of polyethylene terephthalate or polyester. The physical vibration of the foil is not significant to the human eye but does modify the intensity of the received signal, thus implying a small measured value of Rytov variance in the channel under study, however this small deviation and variance is not associated with the temperature effects. On the other hand, the turbulence in the channel 1 was gradually changed from low to moderate conditions.

In Fig. 3(a)
Fig. 3 Measured dependence of (a) Rytov variances in both channels derived from received optical signal and from thermal sensors measurements (symbols, red circles - channel 1, blue crosses - channel 2) and (b) Cn2 theoretical relations (black dotted lines), Cn2 derived from measured thermal distributions via Eqs. (3) and (4) (channel 1 red and channel 2 blue lines) and Cn2 derived from measured of optical power on CT2 measured by the sensor line (symbols; red circles - channel 1, blue crosses – channel 2)
Rytov variance derived either from the fluctuation of received optical signal (enumerated with the correction of the effective area of the photodetector aperture averaging σ12(0)/σ12(D) = 0.69 in accordance to [7

7. M. A. Khalighi, N. Schwartz, N. Aitamer, and S. Bourennane, “Fading reduction by aperture averaging and spatial diversity in optical wireless systems,” J. Opt. Commun. Netw. 1(6), 580–593 (2009). [CrossRef]

]) and from the thermal sensors derived by integration over the thermal distribution using Eq. (5) for the same parameters are presented. Red circles show particular measurements for channel 1 with increased turbulence levels while the blue crosses represent parallel measurements for channel 2. Even though initially there was no turbulence in channel 2, we experienced some deviations in channel 2 due to the flow around foils and its small vibration. As it can be seen, the variance in optical signal increases even though there is no linear dependence with thermal variations within the channel.

Figure 3(b) gives an insight to the ratio of the thermal structure and the refractive index structural parameters. Black dotted lines show theoretical Cn2 dependence derived from Eq. (3) in case of the mean temperatures Te 20°C and 40°C. Colored lines, see inset, represent Cn2 values enumerated from each sensor gap for all turbulence sets according to Eqs. (3) and (4), i.e. based on measured thermal differences and ensemble averaged values (channel 1 depicted in red solid line, channel 2 in blue dashed line) and CT2. Finally, single points, red circles and blue crosses for channels 1 and 2, respectively, show the relations between two measured approaches - CT2 measured in channels via thermal distributions and Cn2 observed through fluctuations of the received optical signal in terms of Rytov variance. Cn2 values from three points of view are therefore compared: theoretical assumption, derivation from temperature fluctuations and enumeration of optical received fluctuations.

3.2. Partial correlation in turbulences at channels

The SelC linear combiner samples the entire received signal through multiple branches and selects the branch with the highest SNR value or the irradiance level, provided the photodetectors receive the same amount of background radiation. The output is equal to the signal on only one of the branches and not the coherent sum of the individual photocurrents as is the case in MRC and EGC. This makes SelC suitable for differentially modulated, non-coherent demodulated subcarrier signals. In addition, SelC is of reduced complexity compared to the MRC and EGC and its conditional SNR is given by:
γSelC(I)=R2A2Imax22Nσ2,
(7)
where Imax = max(I1, I2,…, IN). The pdf of the received irradiance,p(Imax), given by Eq. (8), is obtained by first determining its cumulative density function (cdf) and then differentiating.
p(Imax)=21NNexp(y2)Iσi2π[1+erf(y)]N1,
(8)
where

y=ln(I/I0)+σl2/22σl.
(9)

From the measurements it was observed that the above mentioned analytical assumptions lead to overestimation of received signal deviation in case of two channels crossing non-correlated turbulences. As can be seen in Fig. 7
Fig. 7 Examples from comparisons of measured and calculated Selection Combining diversity with Rytov variance in channels (a) σ12 = 0.0305, σ22 = 1.5606, (b) σ12 = 0.0608, σ22 = 5.4235
from comparison of probability density functions of the measured route diversity data and the statistically derived pdf by Eq. (8) there is higher deviation in the measured selection diversity signal than expected. With increased turbulence levels in one of the channels we experienced heavier tails of pdf. Even though Eq. (8) in majority cases introduces quite a precise estimate it was derived that the combined diversity statistics of the received route diversity signal follow the modified Student's t-distribution with N-degree of freedom (corresponding to number of channels, i.e. in our case N = 2) described by the density function:

p=Γ(N+12)σNπΓ(N20)[N+(II00.1σ)2N]
(10)

5. Conclusion

Acknowledgments

This research project forms part of the both teams activities within the frame of COST ICT Action IC1101 - Optical Wireless Communications - An Emerging Technology (OPTICWISE). Publication was supported by the MEYS CR grant LD12058.

References and links

1.

Z. Ghassemlooy, W. Popoola, and S. Rajbhandari, Optical Wireless Communications: System and Channel Modelling with MATLAB (CRC, 2012).

2.

M. Grabner and V. Kvicera, “The wavelength dependent model of extinction in fog and haze for free space optical communication,” Opt. Express 19(4), 3379–3386 (2011). [CrossRef] [PubMed]

3.

J. Perez, Z. Ghassemlooy, S. Rajbhandari, M. Ijaz, and H. Minh, “Ethernet FSO communications link performance study under a controlled fog environment,” IEEE Commun. Lett. 16(3), 408–410 (2012). [CrossRef]

4.

Z. Ghassemlooy, H. Le Minh, S. Rajbhandari, J. Perez, and M. Ijaz, “Performance analysis of ethernet/fast-ethernet free space optical communications in a controlled weak turbulence condition,” J. Lightwave Technol. 30(13), 2188–2194 (2012). [CrossRef]

5.

X. Zhu and J. M. Kahn, “Performance bounds for coded free-space optical communications through atmospheric turbulence channels,” IEEE Trans. Commun. 51(8), 1233–1239 (2003). [CrossRef]

6.

W. Gappmair, “Further results on the capacity of free-space optical channels in turbulent atmosphere,” IET Commun. 5(9), 1262–1267 (2011). [CrossRef]

7.

M. A. Khalighi, N. Schwartz, N. Aitamer, and S. Bourennane, “Fading reduction by aperture averaging and spatial diversity in optical wireless systems,” J. Opt. Commun. Netw. 1(6), 580–593 (2009). [CrossRef]

8.

T. A. Tsiftsis, H. G. Sandalidis, G. K. Karagiannidis, and M. Uysal, “Optical wireless links with spatial diversity over strong atmospheric turbulence channels,” IEEE Trans. Wirel. Comm. 8(2), 951–957 (2009). [CrossRef]

9.

S. M. Navidpour, M. Uysal, and M. Kavehrad, “BER performance of free-space optical transmission with spatial diversity,” IEEE Trans. Wirel. Comm. 6(8), 2813–2819 (2007). [CrossRef]

10.

H. Moradi, H. H. Refai, and P. G. LoPresti, “Switch-and-stay and switch-and-examine dual diversity for high-speed free-space optics links,” IET Optoelectron 6(1), 34–42 (2012). [CrossRef]

11.

R. K. Tyson, “Bit-error rate for free-space adaptive optics laser communications,” J. Opt. Soc. Am. A 19(4), 753–758 (2002). [CrossRef] [PubMed]

12.

V. Weerackody and A. R. Hammons, “Wavelength Correlation in Free Space Optical Communication Systems,” in Proceedings of IEEE Military Communications Conference 2006, (IEEE, 2006), pp. 1–6. [CrossRef]

13.

J. A. Anguita, M. A. Neifeld, and B. V. Vasic, “Spatial correlation and irradiance statistics in a multiple-beam terrestrial free-space optical communication link,” Appl. Opt. 46(26), 6561–6571 (2007). [CrossRef] [PubMed]

14.

N. D. Chatzidiamantis, A. S. Lioumpas, G. K. Karagiannidis, and S. Arnon, “Adaptive subcarrier PSK intensity modulation in free space optical systems,” IEEE Trans. Commun. 59(5), 1368–1377 (2011). [CrossRef]

15.

S. Rosenberg and M. C. Teich, “Photocounting Array Receivers for Optical Communication through the Lognormal Atmospheric Channel. 2: Optimum and Suboptimum Receiver Performance for Binary Signaling,” Appl. Opt. 12(11), 2625–2635 (1973). [CrossRef] [PubMed]

16.

A. Belmonte, A. Comerón, J. A. Rubio, J. Bará, and E. Fernández, “Atmospheric-turbulence-induced power-fade statistics for a multiaperture optical receiver,” Appl. Opt. 36(33), 8632–8638 (1997). [CrossRef] [PubMed]

17.

M. Jeganathan, M. Toyoshima, K. Wilson, J. James, G. Xu, and J. Lesh, “Data analysis results from the GOLD experiments,” Proc. SPIE 2990, 70–81 (1997). [CrossRef]

18.

F. G. Walther, S. Michael, R. R. Parenti, and J. A. Taylor, “Air-to-Ground Lasercom System Demonstration Design Overview and Results Summary,” Proc. SPIE 7814, 78140Y, 78140Y-9 (2010). [CrossRef]

19.

E. J. Lee and V. W. S. Chan, “Part 1: Optical communication over the clear turbulent atmospheric channel using diversity,” IEEE J. Sel. Areas Comm. 22(9), 1896–1906 (2004). [CrossRef]

20.

N. Letzepis, K. D. Nguyen, A. G. Fabregas, and W. G. Cowley, “Outage analysis of the hybrid free-space optical and radio-frequency channel,” IEEE J. Sel. Areas Comm. 27(9), 1709–1719 (2009). [CrossRef]

21.

E. Lee, J. Park, D. Han, and G. Yoon, “Performance analysis of the asymmetric dual-hop relay transmission with mixed RF/FSO links,” IEEE Photon. Technol. Lett. 23(21), 1642–1644 (2011). [CrossRef]

22.

W. O. Popoola, Z. Ghassemlooy, H. Haas, E. Leitgeb, and V. Ahmadi, “Error performance of terrestrial free space optical links with subcarrier time diversity,” IET Commun. 6(5), 499–506 (2012). [CrossRef]

23.

W. O. Popoola, Z. Ghassemlooy, J. I. H. Allen, E. Leitgeb, and S. Gao, “Free-space optical communication employing subcarrier modulation and spatial diversity in atmospheric turbulence channel,” IET Optoelectron 2(1), 16–23 (2008). [CrossRef]

24.

COST action IC 1101 OPTICWISE Optical Wireless Communications - An Emerging Technology”, retrieved 20.11.2012, http://opticwise.uop.gr/.

25.

A. Kashyap, K. Lee, M. Kalantari, S. Khuller, and M. Shayman, “Integrated topology control and routing in wireless optical mesh networks,” Comput. Netw. 51(15), 4237–4251 (2007). [CrossRef]

26.

J. Libich, S. Zvanovec, and M. Mudroch, “Mitigation of time-spatial influence in free-space optical networks utilizing route diversity,” Proc. SPIE 8246, 82460O (2012). [CrossRef]

27.

S. Kaneko, T. Hamai, and K. Oba, “Evaluation of a free-space optical mesh network communication system in the Tokyo metropolitan area,” J. Opt. Netw. 1, 414–423 (2002).

28.

W. K. Pratt, Laser Communication Systems (John Wiley & Sons, 1969).

29.

S. Karp, R. M. Gagliardi, S. E. Moran, and L. B. Stotts, Optical Channels: Fibers, Clouds, Water and the Atmosphere (Plenum, 1988).

30.

G. R. Osche, Optical Detection Theory for Laser Applications (Wiley-Interscience, 2002).

31.

A. Kolmogorov, ed., Turbulence, Classic Papers on Statistical Theory (Wiley-Interscience, 1961).

32.

L. C. Andrews and R. L. Phillips, Laser Beam Propagation through Random Media (SPIE, 2nd edition, 2005).

33.

Y. C. Ko, M. S. Alouini, and M. K. Simon, “Analysis and optimization of switched diversity systems,” IEEE Trans. Vehicular Technol. 49(5), 1813–1831 (2000). [CrossRef]

OCIS Codes
(010.1330) Atmospheric and oceanic optics : Atmospheric turbulence
(060.4510) Fiber optics and optical communications : Optical communications
(060.2605) Fiber optics and optical communications : Free-space optical communication

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: January 11, 2013
Revised Manuscript: February 18, 2013
Manuscript Accepted: March 11, 2013
Published: March 20, 2013

Citation
Stanislav Zvanovec, Joaquin Perez, Zabih Ghassemlooy, Sujan Rajbhandari, and Jiri Libich, "Route diversity analyses for free-space optical wireless links within turbulent scenarios," Opt. Express 21, 7641-7650 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-6-7641


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References

  1. Z. Ghassemlooy, W. Popoola, and S. Rajbhandari, Optical Wireless Communications: System and Channel Modelling with MATLAB (CRC, 2012).
  2. M. Grabner and V. Kvicera, “The wavelength dependent model of extinction in fog and haze for free space optical communication,” Opt. Express19(4), 3379–3386 (2011). [CrossRef] [PubMed]
  3. J. Perez, Z. Ghassemlooy, S. Rajbhandari, M. Ijaz, and H. Minh, “Ethernet FSO communications link performance study under a controlled fog environment,” IEEE Commun. Lett.16(3), 408–410 (2012). [CrossRef]
  4. Z. Ghassemlooy, H. Le Minh, S. Rajbhandari, J. Perez, and M. Ijaz, “Performance analysis of ethernet/fast-ethernet free space optical communications in a controlled weak turbulence condition,” J. Lightwave Technol.30(13), 2188–2194 (2012). [CrossRef]
  5. X. Zhu and J. M. Kahn, “Performance bounds for coded free-space optical communications through atmospheric turbulence channels,” IEEE Trans. Commun.51(8), 1233–1239 (2003). [CrossRef]
  6. W. Gappmair, “Further results on the capacity of free-space optical channels in turbulent atmosphere,” IET Commun.5(9), 1262–1267 (2011). [CrossRef]
  7. M. A. Khalighi, N. Schwartz, N. Aitamer, and S. Bourennane, “Fading reduction by aperture averaging and spatial diversity in optical wireless systems,” J. Opt. Commun. Netw.1(6), 580–593 (2009). [CrossRef]
  8. T. A. Tsiftsis, H. G. Sandalidis, G. K. Karagiannidis, and M. Uysal, “Optical wireless links with spatial diversity over strong atmospheric turbulence channels,” IEEE Trans. Wirel. Comm.8(2), 951–957 (2009). [CrossRef]
  9. S. M. Navidpour, M. Uysal, and M. Kavehrad, “BER performance of free-space optical transmission with spatial diversity,” IEEE Trans. Wirel. Comm.6(8), 2813–2819 (2007). [CrossRef]
  10. H. Moradi, H. H. Refai, and P. G. LoPresti, “Switch-and-stay and switch-and-examine dual diversity for high-speed free-space optics links,” IET Optoelectron6(1), 34–42 (2012). [CrossRef]
  11. R. K. Tyson, “Bit-error rate for free-space adaptive optics laser communications,” J. Opt. Soc. Am. A19(4), 753–758 (2002). [CrossRef] [PubMed]
  12. V. Weerackody and A. R. Hammons, “Wavelength Correlation in Free Space Optical Communication Systems,” in Proceedings of IEEE Military Communications Conference 2006, (IEEE, 2006), pp. 1–6. [CrossRef]
  13. J. A. Anguita, M. A. Neifeld, and B. V. Vasic, “Spatial correlation and irradiance statistics in a multiple-beam terrestrial free-space optical communication link,” Appl. Opt.46(26), 6561–6571 (2007). [CrossRef] [PubMed]
  14. N. D. Chatzidiamantis, A. S. Lioumpas, G. K. Karagiannidis, and S. Arnon, “Adaptive subcarrier PSK intensity modulation in free space optical systems,” IEEE Trans. Commun.59(5), 1368–1377 (2011). [CrossRef]
  15. S. Rosenberg and M. C. Teich, “Photocounting Array Receivers for Optical Communication through the Lognormal Atmospheric Channel. 2: Optimum and Suboptimum Receiver Performance for Binary Signaling,” Appl. Opt.12(11), 2625–2635 (1973). [CrossRef] [PubMed]
  16. A. Belmonte, A. Comerón, J. A. Rubio, J. Bará, and E. Fernández, “Atmospheric-turbulence-induced power-fade statistics for a multiaperture optical receiver,” Appl. Opt.36(33), 8632–8638 (1997). [CrossRef] [PubMed]
  17. M. Jeganathan, M. Toyoshima, K. Wilson, J. James, G. Xu, and J. Lesh, “Data analysis results from the GOLD experiments,” Proc. SPIE2990, 70–81 (1997). [CrossRef]
  18. F. G. Walther, S. Michael, R. R. Parenti, and J. A. Taylor, “Air-to-Ground Lasercom System Demonstration Design Overview and Results Summary,” Proc. SPIE7814, 78140Y, 78140Y-9 (2010). [CrossRef]
  19. E. J. Lee and V. W. S. Chan, “Part 1: Optical communication over the clear turbulent atmospheric channel using diversity,” IEEE J. Sel. Areas Comm.22(9), 1896–1906 (2004). [CrossRef]
  20. N. Letzepis, K. D. Nguyen, A. G. Fabregas, and W. G. Cowley, “Outage analysis of the hybrid free-space optical and radio-frequency channel,” IEEE J. Sel. Areas Comm.27(9), 1709–1719 (2009). [CrossRef]
  21. E. Lee, J. Park, D. Han, and G. Yoon, “Performance analysis of the asymmetric dual-hop relay transmission with mixed RF/FSO links,” IEEE Photon. Technol. Lett.23(21), 1642–1644 (2011). [CrossRef]
  22. W. O. Popoola, Z. Ghassemlooy, H. Haas, E. Leitgeb, and V. Ahmadi, “Error performance of terrestrial free space optical links with subcarrier time diversity,” IET Commun.6(5), 499–506 (2012). [CrossRef]
  23. W. O. Popoola, Z. Ghassemlooy, J. I. H. Allen, E. Leitgeb, and S. Gao, “Free-space optical communication employing subcarrier modulation and spatial diversity in atmospheric turbulence channel,” IET Optoelectron2(1), 16–23 (2008). [CrossRef]
  24. COST action IC 1101 OPTICWISE Optical Wireless Communications - An Emerging Technology”, retrieved 20.11.2012, http://opticwise.uop.gr/ .
  25. A. Kashyap, K. Lee, M. Kalantari, S. Khuller, and M. Shayman, “Integrated topology control and routing in wireless optical mesh networks,” Comput. Netw.51(15), 4237–4251 (2007). [CrossRef]
  26. J. Libich, S. Zvanovec, and M. Mudroch, “Mitigation of time-spatial influence in free-space optical networks utilizing route diversity,” Proc. SPIE8246, 82460O (2012). [CrossRef]
  27. S. Kaneko, T. Hamai, and K. Oba, “Evaluation of a free-space optical mesh network communication system in the Tokyo metropolitan area,” J. Opt. Netw.1, 414–423 (2002).
  28. W. K. Pratt, Laser Communication Systems (John Wiley & Sons, 1969).
  29. S. Karp, R. M. Gagliardi, S. E. Moran, and L. B. Stotts, Optical Channels: Fibers, Clouds, Water and the Atmosphere (Plenum, 1988).
  30. G. R. Osche, Optical Detection Theory for Laser Applications (Wiley-Interscience, 2002).
  31. A. Kolmogorov, ed., Turbulence, Classic Papers on Statistical Theory (Wiley-Interscience, 1961).
  32. L. C. Andrews and R. L. Phillips, Laser Beam Propagation through Random Media (SPIE, 2nd edition, 2005).
  33. Y. C. Ko, M. S. Alouini, and M. K. Simon, “Analysis and optimization of switched diversity systems,” IEEE Trans. Vehicular Technol.49(5), 1813–1831 (2000). [CrossRef]

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